Geological Survey published the first detailed account of the effects of the flood on the Snake River Plain. The name "Bonneville Flood" first appeared in the literature in Richmond and others, Large rounded boulders of basalt characterize many deposits left by the flood along the Snake River Plain.
Powers, who recognized that these boulders were of catastrophic origin, and Malde applied the name of Melon Gravel to the boulder deposits Malde and Powers, They were inspired to use this term after observing a road sign in that called the boulders "petrified watermelons. At Portneuf Narrows, a canyon 45 miles northwest of Red Rock Pass, the flood is estimated to have reached a height of feet. The release of water from Lake Bonneville was apparently initiated by sudden erosion of unconsolidated material on the northern shoreline near Red Rock Pass.
Although Malde originally proposed a flood date of approximately 30, years ago, he has subsequently revised this age to 15, years ago. Size of the Flood Malde estimated that the probable peak discharge of the flood was approximately one-third cubic miles per hour 15 million cubic feet per second.
This is to be compared with a maximum historic discharge in the upper Snake River of 72, cfs at Idaho Falls in June of The total flood volume is believed to be about cubic miles. The catastrophic flood from glacial Lake Missoula, which swept across northern Idaho, Washington and Oregon, caused far more disturbance than did the Bonneville Flood. When the ice dam failed that contained impounded Lake Missoula, cubic miles of water were suddenly released.
This flood escaped at a peak discharge estimated to be 9. This rate exceeds that of the Bonneville Flood by 30 times. Duration of the Flood Malde suggested that most of the important despositional and erosional features of the Bonneville Flood were developed in a few days; however, the Snake River sustained a high rate of flow for more than a year.
Melon Gravels The Melon Gravels deposited by the flood average three feet in diameter, but some well-rounded boulders range up to 10 feet in diameter. These boulders are composed almost entirely of basalt broken from nearby basalt flows. Only several miles of transportation by the flood was sufficient to round the boulders after which they were dumped in unsorted deposits up to feet thick.
Melon gravel bars are as much as one mile wide by 1. Marginal Channels Many channels were cut by flood waters near the margin of the main channel of the Snake River.
Gilbert established that the lake, with a maximum depth of at least 1, feet, covered an area of about 20, square miles in what is now northwestern Utah, northeastern Nevada, and southeastern Idaho.
He determined that at its highest level, which he named the Bonneville Shoreline, Lake Bonneville overflowed the rim of the Great Basin near Red Rock Pass in southeastern Idaho at an elevation of about 5, feet above sea level and spilled into a tributary of the Snake River, eventually flowing into the Pacific Ocean. He concluded that when these waters suddenly breached the relatively unconsolidated sediments forming the pass, they quickly scoured a channel down to the bedrock and released a catastrophic flood down the Snake River.
This event, now known as the Bonneville Flood, lowered the outlet elevation and reduced the surface elevation of Lake Bonneville in a short time, probably less than a year, to a more stable level at about 4, feet above sea level. Gilbert named this post-flood level the Provo Shoreline. Gilbert noted that the shorelines which formed when the lake was at the Bonneville and Provo levels are now at considerably higher elevations in the central part of the lake basin than they are around its edges.
Gilbert noted that an excess of evaporation over inflow must have drawn the lake down from the Provo Shoreline. His final report on Lake Bonneville was published in as U. Geological Survey Monograph 1. For the next half century very little was added to the understanding of the lake developed by Gilbert.
Since the s, numerous studies using new topographic maps, aerial photographs, new techniques for soil and lake-bed studies, and new techniques for dating sediments and archaeological materials have contributed to a rapidly growing body of information on Great Salt Lake and Lake Bonneville. They have also refined the chronology of major deep-lake events and are leading to a better understanding of many of the lower lake stages that postdate Lake Bonneville.
The climate associated with the most recent major ice age filled the lake to approximately feet above the present Great Salt Lake elevation at what is now known as the Stansbury Level.
This lake covered approximately 9, square miles and its shorelines stand out clearly above the oil refineries near the State Capitol, by the Kennecott Smelter, and immediately east of Wendover. The lake then resumed its rise until by about 15, years ago it reached the lowest pass out of the Bonneville Basin and flowed into the Snake River drainage.
This lake level, the Bonneville Level, was controlled by the height of the pass near Red Rock Pass, at approximately a 5,foot elevation. The immense lake, with a surface area of 19, square miles, left shoreline traces for over 2, miles.
Approximately 14, years ago, a catastrophic flood took place at the natural dam structure known as Red Rock Pass. Increasing water levels and seepage at the dam resulted in structural collapse, producing a foot wall of water spread throughout the Portneuf River Valley and into adjacent valleys along the path. Many geological features found in the flood path are the result of this catastrophic event, believed to be the second largest in known geologic history.
Throughout the semester, GIS Services will continue releasing bi-weekly maps on a variety of topics for the purpose of demonstrating ideas and uses for incorporating geospatial technology into research and projects you are developing.
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